Electrical steel sheets are used in motors, transformers and the like. An insulating coating formed on the electrical steel sheet is required to have various properties such as interlaminar resistance, ease of processing and forming, and stability during storage and usage. In particular, an insulating coating excellent in punchability can reduce the number of times a die used for punching is replaced. An insulating coating excellent in adhesion property reduces the frequency of cleaning due to coating delamination. Therefore, such an insulating coating is easy to handle and excellent in convenience. Properties required for the insulating coating formed on the electrical steel sheet depend on applications. Therefore, various insulating coatings are under development depending on applications.
When an electrical steel sheet is used to manufacture a product, the electrical steel sheet is usually punched, sheared or bent. Working the electrical steel sheet in such a way may possibly deteriorate magnetic properties thereof by residual strain. Stress relief annealing is often performed at a temperature of about 700° C. to 800° C. to ameliorate the deterioration of the magnetic properties. Thus, in performing stress relief annealing, an insulating coating needs to have heat resistance sufficient to withstand heat during stress relief annealing.
Insulating coatings formed on electrical steel sheets can be categorized into three types below:                (1) An inorganic coating that withstands stress relief annealing with a focus on weldability and heat resistance.        (2) A resin-containing inorganic coating (that is, a coating which has inorganic with some organic materials) that withstands stress relief annealing to achieve both weldability and heat resistance.        (3) An organic coating incapable of withstanding stress relief annealing for special applications.        
General-purpose insulating coatings capable of withstanding heat during stress relief annealing are those containing an inorganic component as described in Types (1) and (2). The inorganic component used often includes a chromium compound. An example of a Type (2) insulating coating that contains the chromium compound is a chromate insulating coating.
A Type (2) chromate insulating coating is formed by one-coating-one-baking. The Type (2) chromate insulating coating can remarkably enhance the punchability of an electrical steel sheet provided with insulating coating and therefore is more widely used as compared to a Type (1) inorganic coating.
For example, Japanese Examined Patent Application Publication No. 60-36476 discloses an electric iron plate having an electrically insulating coating obtained such that a treatment solution is applied to a surface of a base electrical steel sheet and then baked by a common method, the treatment solution being obtained such that a resin emulsion having a vinyl acetate/VeoVa ratio of 90/10 to 40/60 as an organic resin and an organic reducing agent are blended with an aqueous solution of a dichromate containing at least one divalent metal in proportions of 5 parts to 120 parts by weight of resin solid matter in the resin emulsion and 10 parts to 60 parts by weight of the organic reducing agent to 100 parts by weight of CrO3 in the aqueous solution.
However, in recent years, electrical steel sheets with an insulating coating containing no chromium compound have been demanded in the field of electrical steel sheets because of rising environmental awareness.
Therefore, an electrical steel sheet with an insulating coating containing no chromium compound has been developed. For example, Japanese Unexamined Patent Application Publication No. 10-130858 discloses an insulating coating that contains no chromium compound and can improve punchability. The insulating coating disclosed in JP '858 contains resin and colloidal silica (alumina-containing silica). Japanese Unexamined Patent Application Publication No. 10-46350 discloses an insulating coating made of one or more of colloidal silica, alumina sol, and zirconia sol that contains a water-soluble or emulsion resin. Japanese Patent No. 2944849 discloses an insulating coating free from a chromium compound that contains a phosphate as a major component and contains resin.
However, electrical steel sheets with an insulating coating containing no chromium compound may be inferior in punchability and adhesion property (adhesion between an insulating coating and an electrical steel sheet) to an insulating coating containing a chromium compound.
On the other hand, for example, Japanese Patent No. 3718638 discloses a method of improving adhesion property by suppressing the amount of Fe in the coating of a polyvalent metal phosphate to satisfy 0≤Fe/P≤0.10. Furthermore, Japanese Unexamined Patent Application Publication No. 2005-240131 discloses a method of improving properties of an insulating coating by suppressing dissolution of Fe into the coating solution, though no particular values are specified therein.
In general, properties of an insulating coating probably tend to be deteriorated by dissolution of Fe into the insulating coating as suggested above. However, in a coating formed such that a paint containing no chromium compound, where chromium compound produces a passivation effect, is directly applied to a surface of an electrical steel sheet and then baked, it is difficult to control the dissolution of Fe. As a result, it is difficult to sufficiently enhance the performance of the insulating coating, particularly, for example, the punchability and adhesion property thereof.
Japanese Unexamined Patent Application Publication Nos. 2003-193263 and 2004-285481 disclose a method of preparing an iron core having end insulation properties at low temperature in a short time. In the method, formation of a siloxane bond network is accelerated by introducing a metal or metalloid selected from the group consisting of Fe, Li, Na, K, Mg, Ca, Cr, Mn, Co, Ni, Cu, Zn, Y, Ti, Zr, Nb, B, Al, Ge, Sn, P, Sb, and Bi into an insulating coating in the form of an alkoxide or a chloride. However, JP '263 and JP '481 do not describe how to accelerate formation of the siloxane bond network in detail in an example or do not describe the particular possibility of improving punchability, adhesion property and the like.
It could therefore be helpful to provide an electrical steel sheet provided with insulating coating excellent in punchability and adhesion property.